A profoundly deaf, hard of hearing or speech impaired person can communicate using a text phone (also referred to as a "textphone") to another text phone user over a regular telephony connection. The user of a first text phone makes a telephone call by dialling the telephone number of a second text phone to establish a telephony connection. When the second user answers the telephone call the first user sends a greeting over the connection by using his text phone to play an audio data signal over the connection. The audio data signal follows a standard audio data protocol (eg v21, v22 bis, v23) which is understandable by the second user's text phone. On receiving the greeting the second user's text phone will send a audio response to acknowledge the connection. After this handshaking the users can have a conversation using text messages entered on the keyboards which their respective text phones convert into audio signals. These audio signals are sent over the connection in a similar fashion to voice conversations.
A problem exists when a text phone user wishes to communicate with someone who does not have access to a text phone. This finds one solution in a relay service such as the National Telephony Relay Service in the UK and equivalent services in the rest of the world. Such services are funded at least in part by the national telecommunication companies and comprise a call center of operators having access to text phone or computer emulation of a text phone and conferencing or multi-line telephony facilities. A text phone user will dial into the service over a first telephony connection and make a request using a text message to the operator who has or is emulating a similar machine to the user. The request will contain the telephone number of a party the user wishes to communicate with. The operator dials this number on a regular telephone based system and opens a second telephony connection when the party answers. The operator may briefly explain that he is a Relay operator acting on behalf of the text phone user. The operator then acts as an intermediary as he receives text messages from the user over the first connection and reads them to the party on the second connection. Conversely the party replies verbally to the operator over the second connection and the operator types text messages and sends them to the user over the first connection.
This solution does have several disadvantages. Chiefly it relies on a limited number of operators to handle the calls and supply is fixed in the short term so that it may be under-utilised in one period and saturated in another such that expensive resources are not used efficiently. Furthermore since this service is a human translation service the costs are relatively high and when, as is usual the service is subsidised, the total number of operators is constrained to below the actual demand. Also such a solution lacks absolute privacy and users may be discouraged in its use for sensitive or business matters.